(1) Field of the Invention
This invention relates to a method and apparatus for transmitting digital serial data. More specifically the invention relates to a method and apparatus for wirelessly transmitting digitized analog data and receiving and reconstructing the data on receipt.
(2) Description of the Prior Art
It is becoming more and more desirable to create wireless radio telemetry links to take advantage of modern advances in sensor technology. Putting the sensors in remote locations allows monitoring of data that was previously uncollectable. In such a situation it is mandatory to keep power consumption to an absolute minimum to allow for long battery life or even the possibility of wireless power transmission. In the latter, power transmission efficiencies tend to be low, mandating the very lowest power consumption for the sensor and its associated electronics. The associated electronics may include a preamplifier, analog-to-digital converter, microcontroller, and RF data transmitter.
FIG. 1A shows the timing of a typical analog-to-digital (A/D) converter, specifically the Analog Devices AD977. This device has a maximum rated sampling rate of 100 ksamples/sec. The sampling speed can be provided externally by an EXT CLK signal 6. With each sample, the device produces a 16-bit serial word. The device can also be programmed to produce a synchronization pulse as shown at 8 in the diagram at the beginning of each 16-bit data packet. This trace 8 is labeled “SYNC” in accordance with the other figures. The DATA OUT signal is shown as trace 10. In prior art systems, a composite waveform is produced from the last two traces 8 and 10 by connecting the separate inputs of an OR gate before modulating a transmitter.
One flexible way of generating the essential control signals to produce a synchronization pulse output is to use a microprocessor. The microprocessor may be a small, 8-bit unit such as the Philips 87LPC764. The start convert and data clock pulses (i.e., the first two A/D timing waveforms) may be generated by the microprocessor with only a few lines of assembly code. Then, the code may be “looped” back for another sampling interval.
The serial A/D output is then used to modulate a typical radio transmitter, such as the Maxim 1472, which will produce a composite, modulated signal centered around a carrier of 315, 433, or 915 MHz. These are popular license-free bands and are used only as examples. The MAX1472 is available as a 315 MHz or a 433 MHz unit and is a complete digital RF transmitter on a board that has only a 1 square inch footprint.
The major challenge occurs upon receiving the signal. FIG. 1B shows the timing diagram of a typical digital-to-analog converter (specifically, the AD5542 by Analog Devices). After the signal is received and demodulated, the received bitstream here called DATA IN 14 should be converted back into the desired original waveform in the D/A converter utilizing the clock signal CLK 16. The fundamental problem, however, is locating the beginning of the data packet because, as received, the ENABLE signal 18 is tied in with the DATA IN signal 14.
A known method of detecting the packet boundary is to send a sync or starting pulse which is much longer than one “high” data bit. A time interval measurement is then performed after detecting the positive-going edge of the sync pulse. A processor would perform these steps and send the result to the D/A converter. However, this is computationally slow and requires large blocks of memory. When a memory refresh in the detection processor is performed, a glitch or missing data point may occur in the detection. This method has the additional problem that a string of high data bits in the data packet could be mistaken for a start or a sync pulse. This potential problem would throw the detection hopelessly out of synchronization.